1. Field of the Invention
The present invention relates to a time slot assigning method and a signal transmission method used in a mobile communications system, and more particularly, to a technique for assigning time slots to an uplink and a downlink for transmitting signals between a base station and a mobile station.
The invention is also directed to a base station control apparatus that controls assignment of time slots carried out at a base station based on the above-described time slot assigning method.
The invention is also directed to a base station and a mobile station, which transmit and receive signals using time slots assigned in accordance with the time slot assigning method.
2. Description of Related Art
(Conventional Technique 1)
In wideband-CDMA (W-CDMA), which is one of the third-generation mobile communications schemes, there are two known systems, that is, FDD (frequency division duplex) and TDD (time division duplex) systems. With FDD, uplink and downlink are assigned different frequency channels. With TDD, transmission signals are multiplexed in a time-dividing manner.
It is expected, in the coming fast multimedia communications, that the traffic will increase more and more on both uplink and downlink, as compared with conventional audio-related services. Since TDD can assign time slots to uplink and downlink channels easily and efficiently, while dealing with asymmetric traffic, the TDD system is a preferable system answering the needs for fast communications in increasing traffic.
In mobile communications, cells of respective base stations are arranged so as to cover the service area without discontinuity in order to two-dimensionally spread the service area. In such a cell arrangement, it is necessary to prevent interference from adjacent cells.
FIG. 1 illustrates interference patterns from the adjacent cell. Base station 101a defines a cell 105a, and base station 101b defines a cell 105b. The cell 105a is subjected to interference from the adjacent cell 105b. Interference is categorized in four patterns (indicated by the circled figures in FIG. 1) listed below.    1. Interference at base station 101a received from the base station 101b of the adjacent cell    2. Interference at base station 101a received from the mobile station 103b located in the adjacent cell    3. Interference at mobile station 103a received from the base station 101b of the adjacent cell    4. Interference at mobile station 103a received from the mobile station 103b located in the adjacent cell.
Among these, pattern 1 is the most severe because the desired wave from the mobile station is weak, while the wave from the base station is strong. In addition, the wave from the base station is unobstructed because in general base stations are set up at a higher place. If the mobile stations 103a and 103b are located close to each other, interference 4 may also become serious. In a W-CDMA TDD system, the base stations included in the system synchronize with each other so as not to produce interference adversely affecting adjacent cells. In other words, the operations of the base stations have to be controlled so that the signal transmission timings are the same among all the base stations.
(Conventional Technique 2)
The above-described conventional W-CDMA TDD system is capable of preventing interference patterns 1 and 4 because all the base stations in the system take the same transmission timing. However, this system is incapable of dynamic time-slot assignment of uplink and downlink for each cell. For this reason, the conventional TDD system cannot flexibly cope with changes in up and down traffic that vary in a dynamic manner. To solve this problem, HIPERLAN (high performance radio LAN) Type 2, which is one of the radio LAN standards defined by ETSI (European Telecommunication Standards Institute), has been proposed. HIPERLAN Type 2 employs a DFS (dynamic frequency selection) technique that monitors multiple frequency channels and carries out communications using unused channels. With HIPERLAN Type 2, different frequency channels are used in adjacent cells, and consequently, interference pattern 1 shown in FIG. 1 can be prevented. At the same time, assignment of time slots for the uplink and downlink is controlled dynamically for each cell.
(Conventional Technique 3)
Improvement of a W-CDMA TDD system for dynamically controlling uplink and downlink time slots has also been proposed. An example of such improvement uses a punctured code on the downlink, and assigns uplink slots in the erase bits. See Miki, Sanbe, and Morinaga, “Study of Uplink Slot Assigning Control for Up/down Asymmetric System Using DS-CDMA/TDD”, Shingaku Gihou RS2000-259. With this proposal, punctured codes are applied to the down signals, and their erase bits are arranged in time slots. A portion of the erase bits are assigned as uplink slots depending on the uplink traffic.
In a mobile communications system employing W-CDMA TDD, if transmission timing for uplink slot and downlink slot differs among different base stations, serious interference occurs on an uplink signal from a mobile station to a base station in a cell due to signal transmission from the adjacent base station 101b through the downlink. The above-described conventional technique 1 avoids such interference by synchronizing the uplink and downlink transmission timings at all the base stations. However, this system can not change the ratio between the uplink slot and the downlink slot in the communications environment where the traffic condition differs among cells. It is difficult for the mobile communications system using conventional W-CDMA TDD to assign resources in response to asymmetric changes in traffic on the uplink and the downlink, and therefore, satisfactory channel efficiency can not be obtained.
HYPERLAN Type 2 described in the conventional technique 2 can solve the above-described two problems by changing the frequencies of adjacent cells. However, this technique is not practical because it requires a number of frequency channels (and therefore, a broad frequency band) in order to avoid mutual interference in a mobile communications system defining a two-dimensional service area.
The W-CDMA TDD system using a punctured code described in the conventional technique 3 causes throughput to decrease because a convolutional code with a low encoding rate is applied to the downlink signal in advance. In addition, this system only prevents the interference on the downlink signal.